Four-wire terminal transmission control



Jan. 1, 1935. D. MITCHELL 6 FOUR-WIRE TERMINAL TRANSMISSION CONTROL Filed Oct. 6, 1931 INVENTOR W f ATTORNEY sessi e I q, p TERMINAL 1' Tmsms SIGN; o 1 'f OO R'O L 'Doren MitchellfMartinsvillQN l ft Sistine! American Telephone and Telegraph a'corporati'on-of'New York r This inyenti'on relates to two-way t or the transmission of 1 energy, suchas telephone systems, over whichlare transmitted the electrical waves representing speech. 1More tip'articu larly, the invention relates to four-wire telephone circuits having terminal arrangements .for controlling the transmission thereover. With greater particularity, the invention relates to four-,wire

telephone circuits haying a terminal connection to -an extending twoewire line and equipped. with voice-operated transmission control apparatus; 1 i- In the: usual terminal arrangement of "afourwire circuit connected to an extending two-wire line, the transmitting"1.1path,is gadapted for transmission from the twoewire, line, and the receiving? path is adapted fortransmissionto that line. These. two' paths of l the f our-wire circuit are connected-to the twe -wire linethrough a hy (bridi coil, and. the impedance of :the twowire line is imperfectly balarioedby an artificial network.v

- The transmittingypathof the fOllTrWiI'B' circuit is usually maintainediinhaunormalg, or noneper:-

'1 iating, condition of disability, ,inr order thatisingi ing may bewprjevented; arrangements-,are then provided: whereby voioel energy 1 in the. transmitting path causes the removalof this disability and thesimultaneous disabling. of thegrec'eivingtpath.

Since the balance between the itwo'wire line and. the artificial network is :not perfect, when energy travels to' the two-wire lineyover the reg cei-ving'path of the four wire circuit, a part: of this tp aratus responsiye to waves in; the transmitting energy in energy is usually reflected over thegtransmitting p as e -i; A ns cet e b ve-memion d' K balance is variable, the valueofthis echo energy I; the value of ,the reflectedfor echo will ary.

e s shem s hisen y ma pa e o 1 false operation of the transmissionscontrol ap- P hi V t i Since; the energy, in the transmittingrpath which represents the-Iyoice of the talker connected to the two-wire linenot onlyuclears the transmitting path but fofwt he four wire circuit, itbecomesnecessary to provide means whereby the distanttalker whose voice .energy has passed over the system tothe receiving path at the near terminal can take control of the i system without interruption" due to later'garriying energytravelingjin the opposite dimotion. The' means i'orfi'giying flsuch control usuallytakes the form of apparatus responsive to thereceiving' pat which, upon*opera tion, disables the transmission control apparatus responstve to energy in" the: transmitting path. It may he, however, that the distant end of the also disables'lthe receiving path i that is,j a link which is noisy or unstableand from whichlinteriering eherg'yxmay' be transmittedsuch as a radioilink. 1 1111811011 a: case,-.the're is the possibilityofsfalsecperation of. the transmission 5 icontrot apparatus responsive'to. energy in the receiving; path eithe; four-mirecircuit; because of the noise (or other? interfering energy-,J which is usually of comparatively sustained.- volume: or slowly yaryingovoiume imconti astrwith the voice to ienergw which is ottrap'rdlytfl'uctuatimg' volume a It is, importantthatmhetransmission control apparatus. "operated from gthea transmitting path shalli respond; to weak, as well as strongyoiceenerg'ys-vto: the speech; of, talkers whobhaye com-.- paratively lowrnabtl-ity to operate such apparatus :as well :as m-thespeech of. talkers withhighability for such operation Qnthe other hand,- the proper precautions must. bevtaken; 'as indicated above, against the false'operati'o n of this appara- 2o tus'; bythe; reflection: marsh in the receiving pa h: F fi fi r h sa war t a r rta t h t {the maximum desirable volume bedeli-verect over Thelappl'ieants inyen tion resides "intour-jwire circuit-terminal. arrangements which, i nyiew of the various problems and requirements outlined above. sire satisfa o .1 n m ss n including .satismtwmne ive. o e a on o h m r s RQR W JDW ulflfifi m i :1; L 1

- the nti n thea ius oi W .o t o o (the tran mis on. on o e psr t se ra i hom-t s' mmmit me pat wo -s9 met h l p e ews w fi nt fami ie ent, the -"operathe o ject ofthefinyention'jis to control erases troi nthe path by" incoming static 0 Furthermore; 5 it the object of the invention to eroviee eermiglamrmngem ents which so enmi venous-means for aneompii'smgtne soyeral"ohieets statectaboye that 'a genefallyj satis tmsqndlproeventiq ofeth l e n so factory condition of terminal transmission control is reached.

' grammatically, and, inlpart schematically, a desirable-circuit 'arrang ernent jlembodying the invention.

With reference to the details of the drawing,

there is disclosed one terminal of a four-wire circuit which is connected toan extending twowire line L. The four-wire circuit includes 'aone-way path L1, which may betermed thetransmitting path and is adaptedrorit a sm sion,

from the two-wire line L, and'a second one-way path L2, which may be termed the receiving path and is adapted for transmission to-the two wire line. The paths L1 and L2 are connectedto the line L through a hybrid coil, and the impedance ofith'e line 'L- is imperfectly balancedby an artificial-networkm' f -51 1; g a The -transmitting==pathi -L1 is normally. disabled at the point a for-the purpose of: the suppression of sin'ging. "Connected across, the path L1: at the point bziis1an5amplifier-detector 'ADi,

which indirectly 1 controls the: operation of a relay- SS :designed to 'removeii the: disability 'from the path: L1 at point a,-.and-a relay ES, which is designed toi disable the :receivingpath L2v at point c. Connected- "ac'ross -the 1 path L2 at the point" d is an' amplifier-detector Y AD2, which controls the operation'of a relay RS- designed to disable the output circuit" of the amplifier-detector'ADn"As-is'well-understood in the art, when voiceen'ergy; for instance, "travelsin the transm'itting path L1 a the relay SS" normally "operates i to to clear the path 'L1- at point w for the transmission of the voice waves, "and 1 the relay ES disables the path'L'2 at Sp'oir'itc to prevent echoes. If, however, voice 'energy coming' from the right end sthe rcur=wire {circuit overpath L2 passes pointfc, it= causes "th'e operation of -the relay RS and the disabling of the output circuit of" amplifier-detector ADi, and later arrivir'fg energy' in the "'ansmitting path Li "crating-relays ssand ES: 4 7 1 Volume 1 control in" the "transmitting path L1 'is effe'cted by means of a'three-electrode vacuum tube'Tr; -which" forms-'"a -bridged impedance variable with the variations of tne' voltage impressed on the" grid of the tubex Likewise, volume conprevented from ptrcl in th 17cc Eris effdtdbYmean's 'of ft three-e1 tipae'j vacuum tube -Tz, which for a b 'dged impedance variable with *'varia'- tion Ofth' v o'ltag sp ts-grid; In addition, r reiving' :path L2 ,rnay be ,efiectcd by meansof th three-"electrode 1v i w t i rms T i s 'i di e variable"as'indicated' 'abov. r j tii 'rhewm egfmgin t e introduc d through 'tub T; d

. -Qt; ece v d;yo ume is ne sa y q h v t -Q Se'QPQ Z3: Q of; .amp ne ector' D For instance, this volume regulation, may be ,so; ad- ;iustedw a h rer-isu Q 1 educ i n of rec v ol me unt l the-cond tmn lea h d-w alseoperation of ,th transmission conhreaten I tmlnof;amp ifi r-de e tor, with; a.; t lfm a- 1 1 i -l rsfera b ii' h thatfthe loss introduce transmitting path L1 a I v ;brid.-coil. It .will be understood that'energy balance between the line L and the network of 20 decibels (db).

The adjustment of the voltages on the grids of tubes T1 and T2 is effected by two potentiometers P1 and P2, which are preferably tied together for simultaneous operation, as is indicated in the drawing. With the specific circuit arrangement disclosed, if the talker connected to! line L has a weak voice, or'a voice of low ability to operate the transmission control apparatus, the arm of potentiometer P1 is turned in the clockwise di- .."rection. .This'operation increases the normally considerable negative biasing voltage impressed on the grid of tube T from the battery B1. Such increase .of negative: voltage raises the bridged impedance of the tube T1 and decreases the loss tentiometer P1 and tube T1 and between potentiometer: P2 andtube T2, respectively,serve-:to prevent clicks in the transmission paths L1 and L2, respectively, as the resultof sudden changes in the settings of thepotentiometers.

It may be desirable-to make suitableadjustments affecting H directly or. indirectly the operation of the transmission .control relays SS and ES when false operation'of these relays is threat ened with terminal balances which are lower than '20 db., for instance. For this purpose the applicantconnects in the output circuit of the amplifier-detector AD1 four polar .relaysSi, S2; S3 and S4; this relay chain vco-operates under certain conditions with-a marginal relay-MS. V r

Relays S1, Sa sa and S4 haveoperating wind-'- ings'w1, 202, L03 andw4, respectively, which are-directly in circuit with the output of amplifier-detector AD1. I Relays S1, S2 and Sa-have, in addition, auxiliary windings wi'ywz and L03, respectively. Furthermore, relays S1, S2, S2 and S4 "may bebia'sed bywindings 1.01",2122, tug and K wi, respectively, The turns of these biasing windings are disclosed in the drawing so as to suggest graphically the relative sensitivities of the' four relays; relay"Sris'more sensitive than,

. ingwhich indicate the direction of pull exerted by each winding on'the associated armature.

' The marginal relay MS hast'wo windings W1 and W2. The winding'W -is under the control of an amplifier-detectorAD, connected'between the hybrid coil and the balancing network; The winding W2 is under'the control of amplifierdetector AD which is connected across the traveling toward the four-wirecircuit in the twowire line L divides at the hybridcoil so that in response to such energy only amplifier-detector AD willbe operated. If,=however, energycomes in over the receiving path L2 of the four-wire circuit; divides at the hybrid: teatime reflected back .from theline-Li'oven: 'ttinfi' p the fiour-wirecircuit; both .of the amplifier? detectors AD andADf: will beaopratedr;

, it .Inrorderthat. the proper ioperatiomofthese two amplifier-detectors may; beessured; it is desirable to provide for adjustment ot: .their :respective sensitivities by thaoperation pi potentiometer 1P1. Forfthis purpose three-electrode; vacuum; tubes T and T are connectedzinrtheinput; circuits of am.- plifier-detectors and-AD! ,1 respectively. 2; Negative biasing voltages are .impressedonthe grids lhlw ichc nt o s V as -...I ..s o vund r o ha -rela ,Sz. w

Qt th t by th a r Bt wm h iurni hes the biasing voltage for the. ;tube;Tl,; and the .bias is. il-d ted y wion iqnzotth ten qme e a n; in n th.L1-.=.. Sh u d .b edth th nd W2 1 elax-Msa h u be made of equal sensitivity wit sensitive,.wil'l be .operated,-while relays which is less. sensitive; than S2,: also remains u operated. h compa t v l w ak ener y n at Li c s h n re ati n 0t wi d ng W fthe a inal relay MS and the armature of this. relay ispulled downward. A. circuit is ;.now completed over hatt r h arma u rq tre MS a e t n path L1 which is hello the armatureoi relay snithearrna-ture of; relay S1,

and a condenser C4; andthe. condenser is charged. A three-electrode-vacuum tube Tssacts as a valve in the circuit including-theauxiliary windings i', 22'. a p i y- 81 Stand Ssre pe ti-vely.. Itwillbe understood; that ithis tcircuitz is normally so adjusted that any ,force exertedby any one of these auxiliary windings will be insufli-j cient to. cause operation by itself of the; relay of which the winding. forms a part., 1 "When, however, condenser 64 ischarged through resistancelRz hy the completion of the above; described circuit, the normal negative voltage applied L to the gridof tube T is reduced-or partly cancellecl-,;; and-an ap preciable amount of; current will fiowvthroughthe circuit including the auxiliarywindingsn; zAs is indicated in the .drawing-,';this current 4 auxiliary windings.aidsthecurrentflowing n n i e a tw ndin ether words, the sensit y of the relays S S2 and .S3 i a di a ae r sitivity??? course, renders tth transmissi control appae ratus responsive to; n'ergy in. nsrnitting tude and enables aLWq I t I L1 at point. a and todisablethereceiving path L2 at point 0 through the operation of rela'yfSa he e u ta s asons: l ys lsem E?- In connection with the circuits just discus ed,

the resistance R4, associated .w th. condenser. C5;

should be quite largefin orderlto provide for the slow leakage of thechargebrittle-condenser upon the termination of energy' transmissi in the path L1. It should be notediiirther a since relay S1 is less sensitive tha relay the above described adjustment pffsensitivity willfgive some margin against clipping. h 1

When energy -corning-f iri ov v he receiving path In of-the feur wire-circuit is 'o "such magnitude'and the balance-betweer'i thef wireflline-h and the artificial networlis siici-r mmsiseepera and condensers Ci and C1. energy" in -path L1 is of appreciable magnitude;

tion ofvrelays SSuand ES; is threatened, it becomes necessarysor desirable. .to introduce an? auxiliary regulation of loss in the path Lz. .The loss introduced by the, bridged: impedance of the three:

electrodeyvabuum tube T2 isdetermined by the valuelth'anthe energy operating on thegwinding W1 by an amount depending onithe balaricebetween the line L and thev network; When the armature of relay MSL pulled up ward', a circuit isprepared through battery, thearmature of relay MS, the armatureof relay S3 the resistance R1, If the reflected relay-S; will operate "to complete this charging circuit fo r' condensers C1 and C and these condensers 'willnbe charged throughresistance R1 and an-inductance The-eftectof this condenser charging is to reduce or partly cancel the negative bias onthe grid of the tubeTz'. This reduction ofibias reduces the bridged impedance and increases the loss introduced at this point inthe receiving path L2. Since relay S3 is more sensitive than relay S2, some margin is' obtained against false operation of "the transmission control through the operation-of relay S2. Since relays Szand S3' areacted on simultaneously "byany change of voltage on the grid of tube T5, the iii-i5- ference in sensitivity between these two relays is maintained constant; It will be understood, 'of

course, that if the energy m the receiving path L2 causes the operation 'ef'reIay-RS through the energization of the operating winding w controlled by amplifier-detector ADQ the'output circuitof amplifier-detectorADi is disabledand no control is had response to later arriving energy in path Li. Y

remains to described-the seaside: "the applicants "terminalarl'angement which provides iorthe prevention of false operationofthe re-- lay RS-designed to disable: the-output of amplifier cletector AD; by noise or? other v interfering energy comingxin over the path L2. For this purposethe applicant employs a proportional desensitizing circuit connected-i across the input of amplifier detector'ADzfas is indicated in the drawingyto'gether withdertain other apparatus saw to be discussed.

The"proportional desensitizing circuit comprises a plurality of vacuumtubes includin'gthe tubes Te and T4; Thprinciple o'f'this circuit and the "fopera tion thereof are clearly disclosed andfu llyydiscus sedin United States Patent No. 1 ,7'72 ,55I,]to Mitchell and Silent, and accordingly, a .detailedldescription ordiscussion herein is hnnecessary, It is suflicient 'to state that the desensitizing circuit distinguishes between "the rapidly fluctuating voice energy and noise energy or the like which is of comparativelysustained volume; The circuit operates "to prevent any appreciable e'fiect upon th apparatus which it controls-by the voice energmbutto" produce an appreciable efiect onsuch apparatus in response to the energypf comparatively sustained volume.

This effect is the variation of the plate current in tlieztubes T3 and T4' (which :are connected in parallel); in accordance with'and'innected in the plate circuit 'of thetube T4 of the sequent,undesired-disabling of the in a clockwisedirectiqn tofldjust for a weak talkdesensitizing circuiti;.'When noise energyo'r the likeicome'sfin over path L2,. a:.bias is :gradually built 'up: inrelay Rszthrough the energization of winding :w': and .this bias is suflicient to prevent the operation .of relay. "RS even though 7 there is fullenergizationpof the operating winding w connected in thecoutputcircuitzof amplifier-detector AD2..:. In'otherwords, ;the relay-RS is" desensitized-1m accordance; with,v the noise" energy,

or other :energy ..of: gcomparatively sustained volume, torthe pointwhich preventsthe operation of" this ,,relay;'by-;-,noise,,or the likeand theconoutput circuit of amplifier-detectorADL;

Since thereduction of thesensitivity of relay RS carries with it an increase of the chance of .false operation, of the transmission; control relays SS-,;and ES by reflected energyof sustained volume in path L1 .(as this energy may; have passed point d path Liz without operating relay RS, and in; its refiectedform-mayoperate the amplifier-detector-h-D ,--it is .desirable to introduce' auxiliaryadjustment ofthe loss introduced in path Lz by the-tubeTzs It will benoted that a resistance r R is included in the plate circuit of tube II: of ,the ,desensitizing device; This resistance may be variable togive a suitable :addi- .fional adjustment.- As. the energy-oicomparatively sustained volumecauses the operation of the desensitizingcircuit, a voltage is built up across resistance. R, or the effective portion thereof, whichtends to cancel the negative voltage'qapplied to the grid of tube T2 by the battery B2, ,-This auxiliary-voltage reduces the impedanceo'i-thegtube T; andflincreases. or tends toincreasethe; loss;;,introducedinpath L2. Thus, if the armsof potentiometers P1 and P2 are turned enthe volumain' path Lz is flowered sooner if the noise cominglin over the path L2 is high thanif such noise is ,cqmparativelylow, .It will bev noted, however, that no penalt I is'attached to a strong talkerconnected t'ojline lli reasonof this auxiliary adjustment, since for such a talker; the proper setting oipotentiometer P2 (and P1) keeps thecancelling voltage applied to the grid of tube T2 low enough to render it ineffective against the normal-negativebiasing voltage on the grid. The applicants 'present invention contemplates the cornbination orvarious combinations of devices or circuit arrangements eachof which may by itself accomplish valuablefadjustme'nt relating to transmission control. 1 Certain of the elements goi11g',to make up such combinations are disclosed andiseparately claimed in one or more of. the {following copnding applications:

'SerialfNof. 559,079'ffiled :Augustj24', 1931, and .seria1 1N0; 567,3g fi1ed October 6,. 1931, infthe na e t me applicant, a Serial No. 560,216

' filed August 29, 1Q3Linthenamesof S. B. Wright andthe. applicant (assigned to the assignee of they present application) Y WhileQthe mvent ongha been disclosed spe cifically for thepurpesa f illustration, it is to be understood fjthatlI such f disclosure does not necessarily limit the scepter the invention, which lete imed. a iliea ended :Q aims.-.

two-wire 1ine,-areceiving path adapted for'transmissionto said line, transmission control apparatus normally responsive-to energy-in said transmitting path of a predetermined or greater magnitude, adjusting means responsive to energy in said transmitting path of less than the predetermined -magnitude for rendering said transmission; control apparatus responsive to such lower energy, and means for preventing the operation of saidadjusting means'in response to reflected energyin said transmitting path, said last-stated means, however, at the same time permitting-the desired operation of said adjusting means in=re 'sponse to original energy in said path.

2. In' a four-wire circuit having a terminal connection to an extending two-wire line, a transmitting path adapted for transmission from the two-wire line, a receiving path adapted for trans mission to said'line, transmission control apparatus'normally responsive to energy in saidtransmitting-path of a predetermined or greater'mag nitude, adjusting means '-'responsive to non-reflected energy in said't'ra'nsmittingpath of less than the predeterminedmagnitude for rendering said transmission control means responsive to such lower energy,'and means responsive to energy in said transmitting path which is a reflection of energy in said receiving-path for adjusting the level of energy .in said receiving pat a 3. In a four-wire circuit havinga terminal connection to an extending 'two wir'eline, a-tr'ans mitting path adaptedfortransmission from the two-wire-line, a receiving path adapted for transi mission to'said line, transmission control apparatus normally responsive to energy said transmitting path of a predetermined or greater magnitude, meansresponsive toenergy originatingin said two-wire line 'and'tra'veling over said transmitting path of less thanthe predetermined mag nitude for'rendering' said'transmission control apparatus' 'responsive to such lower energy, dis abling means responsive to energy in said receiving path for preventing the operation of said transmission control apparatus, means respon sive substantially only toenergy in said receiving path. of comparatively sustained volumeior desensitizing said disabling" n1eans, means responsive to energy said' transmitting path which is a reflection of energyfn saidreceivin'g path foradjustin'g theloss insaid receiving path, and meansresponsive to energy in said receiving path of comparatively sustained volume for ad justing theloss' in saidpath. I

I 4. "In afour wirecircuit having a terminal connection to an extending two wir e line, a transmitting path adapted for transmission from the two-wire line, a receiving path adapted for trans missionto' said line, transmission'control apparatus normally responsive to energy in said transnritting path of a'predetermined or greater magmeans responsive sulSstantially only to energy in .said receiving path of comparatively sustained said receiving path, and means responsive to energy in said transmitting path which is a reflection of energy insaid receiving path for cffecting an auxiliary adjustment of the loss in said receiving path.

:5. In a four-wire circuit having a terminal connection to an extending two-wire line, a transmitting path adapted for transmission from said two-wire line, areceiving path adapted for transmission to said line, transmission control apparatus normally responsive to energy in said trans mitting path of a predetermined or greater magnitude within certain upper and lower limits but substantially above the lower limit, adjusting means responsive to energy in said transmitting path within said limits but of less than said predetermined magnitude for rendering said transmission control apparatus responsive to such lower energy, and means for preventing the operation of said adjusting means in response to reflected energy in said transmitting path, said 10 last-stated means, however, at the same time permitting the desired operation of said adjusting means in response to original energy in said path.

DOREN MITCHELL. 15 

